1. Field of the Invention
The present invention relates to a tungsten forming process, and more especially, to a method using chemical plasma treatment to improve the tungsten surface property.
2. Description of the Prior Art
In the electronic and semiconductor industry, conductive materials are applied to be the electrodes of designed devices and the contacts or connections between them. As one of the widely used conductive metal materials in modern semiconductor integrated circuit technology, tungsten is employed in variety of semiconductor structure, including contact barriers, MOS gate interconnects, and so on.
In comparison with other high electrical conductivity metals like aluminum, tungsten is not the most preferred material for conductive layer such as electrodes or interconnects due to its higher electrical resistivity. With the benefits of low electrical resistivity and compatibility with the matrix substance of silicon dioxide, aluminum is emerged as the most important material for the application of interconnects. But aluminum metallization for interconnects suffers from its inability to withstand high temperature processing, which precludes its use in self-aligned MOS processing. This is not the case for tungsten. The applicability of tungsten to VLSI interconnect application has been considered, and extensive efforts have been directed towards developing the chemical vapor deposition (CVD) of tungsten thin film for such application. Processes for forming CVD-tungsten films both in selective and blanket deposition modes have been successfully pursued.
The chemical vapor deposition of tungsten is generally performed with the well-suited source gas of tungsten hexafluoride, WF.sub.6. Tungsten hexafluoride can be reduced by silicon, hydrogen or silane. The reaction equations are list as follows. The silicon reduction is given by EQU 2WF.sub.6(g) +3Si.sub.(s).fwdarw.2W.sub.(s) +3SiF.sub.4(g) (1)
The hydrogen reduction is given by EQU 2WF.sub.6(g) +3H.sub.2(g).fwdarw.W.sub.(s) +6HF.sub.(g) (2)
The silane reduction is given by EQU WF.sub.6(g) +SiH.sub.4(g).fwdarw.W.sub.(s) +SiF.sub.6(g) +2HF.sub.(g) +3H.sub.2(g) (3)
or EQU 2WF.sub.6(g) +3SiH.sub.4(g).fwdarw.2W.sub.(s) +3SiF.sub.6(g) +6H.sub.2(g) (4)
In addition, tungsten hexafluoride can be reduced by Al and Ti through EQU WF.sub.6(g) +2Al.sub.(s).fwdarw.W.sub.(s) +2AlF.sub.3(g) (5) EQU WF.sub.6(g) +3Ti.sub.(s).sub..fwdarw.W.sub.(s) +3TiF.sub.4(g) (6)
In practice, the process is more complex, with several intermediate reaction products such WF.sub.4 and WF.sub.5 involved. Other source gases such as WCl.sub.6 have also been employed in the hydrogen reduction.
CVD-tungsten appears to be an excellent candidate material for interconnect applications because of its advantaged properties of low resistivity, low stress, and so on. For example, tungsten has a high melting point at the temperature of about 3410.degree. C., and it makes the following high temperature processes possible. The thermal expansion coefficient of tungsten, which closely matches that of silicon, is also a benefit, because it can reduce the stress between tungsten film and silicon. Moreover, tungsten has good electromigration resistance and can form low resistance contacts to silicon. Tungsten has none of the stoichiometry control problem, and that often plague silicides. Most important, tungsten deposited by chemical vapor deposition (CVD) exhibits excellent conformal step coverage, which is a serious problem to the better conductor, aluminum.
Due to the excellent deposition conformability, the most successfully commercial application of tungsten is contact hold filling, which is usually referred as metal plug process and wherein sputtered aluminum is not suitable because of its poor step coverage. In addition, tungsten films have found application as bottom electrodes of the capacitors of dynamic random access memory (DRAM) cells and low-resistance gate interconnections. Moreover, contact barrier materials and ohmic contacts are also considered uses for tungsten.
With all the advantages mentioned above, tungsten still suffers from surface problem when it is used to be the materials of electrodes or interconnecting lines. Although the grain size is not large, tungsten grows into ragged morphology that is shown in FIG. 1, wherein illustrating a semiconductor substrate 10 and deposited tungsten layer 20. As can be seen, the surface of the tungsten layer 20 consists of the as-grown crystal facets with no preferred orientation everywhere. Spires is formed all over the surface and will enhance the local electric field, which results in decrease of the breakdown voltage and increase of leakage current for the conductive layer. Furthermore, rugged surface has poor reflectivity and will cause patterning difficult due to the random reflection of light during the photolithography process.